This invention relates to a current limiting fuse, and more particularly to a slit type current limiting fuse for limiting an overcurrent by having a fusible member disposed in a slit formed in an electrically insulating member or between electrically insulating members to cool an electric arc struck upon the blowout of the fusible member and also to prevent the electric arc from scattering and increasing in cross sectional area thereby to ensure a sufficiently high arc voltage.
Current limiting fuses are frequently employed for the purpose of protecting a wide variety of electrical equipments and lately have been popular as protective means for ebullition cooled semiconductor devices.
Conventional current limiting fuses have generally comprised the fusible member disposed within an arc-extinguishing material such as silica sand and have been operated to limit and interrupt the particular overcurrent by blowing out the fusible member with the overcurrent and extinguishing an electric arc struck simultaneously with the blowout of the fusible member as a result of the electric arc scattering into and cooling by the arc-extinguishing material. With the arc-extinguishing material formed of silica sand, the same is granulated and therefore porous as a whole. In other words, the granulated arc-extinguishing material includes a multitude of minute interstices formed among granules thereof. Accordingly, an electric arc struck upon limiting the particular overcurrent is permitted to scatter into the interstices in the arc-extinguishing material to increase in cross sectional area thereof. This increases in cross sectional area of the arc suppresses a rise of an electric voltage with the result that the result that the excellent current limiting performance can not be obtained.
In order to improve the current limiting performance, it has been already proposed to lengthen the fusible member. However, since the fusible member is surrounded by the sand-shaped arc-extinguishing material which is not very good in thermal conduction, the resulting current limiting fuse has exhibited its own thermal resistance which is greatly affected by a thermal resistance of a mating fusible member. Accordingly, if the fusible member is lengthened then current limiting fuses per se have increased not only in volume but also in thermal resistance. This increase in thermal resistance has imposed a limitation upon the current capacity of current limiting fuses.
Also sand-shaped arc-extinguishing materials are fused with the electric arc struck upon limiting and interrupting an overcurrent to form fusion products. If a plurality of fusible members are disposed close to one another then fusion products built up around each of the fusible members are overlapped or superposed on those built up around the adjacent member. Thus electric arcs cannot be sufficiently cooled, resulting in a decrease in current limiting and interrupting performance. For this reason, conventional current limiting fuses have been required to include the arc-extinguishing chamber having a volume large enough to provide the satisfactory current limiting and interrupting capability even upon a built up of fusion products attendant upon an increase in cross sectional area of the electric arc. This has resulted in the disadvantage that current limiting fuses per se become large-sized. In addition, the current limiting and interrupting performance changes greatly with a filling rate of an arc-extinguishing material involved resulting in the necessity of controlling strictly the filling rate.
In order to solve such problems as above described, there have been recently developed current limiting fuses of the type comprising a solid electrically insulating member having a slit formed therein and a fusible member disposed within the slit to cause a flow of current therethrough. For example, Japanese patent publication No. 45782/1976 discloses and claims a current limiting fuse characterized by a band-shaped flat fusible member put in intimate contact relationship between plate-shaped high heat conductivity, arc-resisting, electrically insulating members from both surfaces thereof, said insulating members extending in a direction orthogonal to said fusible member, and a current limiting and interrupting slit formed by interposing the electrically insulating material between peripheral edge portions of said plate-shaped insulating members except for portions thereof intimately contacted by said fusible member. In current limiting fuses of the type referred to, an abnormal current flows through the fusible member to blow it out thereby to strike an electric arc. Under these circumstances, the electric arc is cooled by the solid electrically insulating members surrounding the same while a cross sectional area thereof is scarcely increased because the electric arc is confined in the slit having a volume defined by the surrounding electrically insulating members. As a result, a corresponding arc voltage exhibits a high rate of rise and also reaches a high magnitude resulting in the excellent current limiting characteristic. Even for the current limiting fuse of the cited Japanese patent publication, however, the current limiting characteristic has not always been satisfactory because a slit width is not considered. Also problems have been left unsolved as to how current limiting fuses of the type referred to are constructed to withstand high voltages and have high current capacities which has been seriously taken in recent years.
Further, in conventional slit type current limiting fuses comprising the fusible element and the slit for housing the latter both being partly or entirely exposed to the ambient atmosphere, various problems have been encountered in that shock sound is generated upon the blowout of the fusible member, high temperature arced gases contaminate adjacent components, a shortcircuit occurs across electrically charged parts adjacent to each other and so on. In order to solve the problems concerning the shock sound, contamination, shortcircuit etc., it has been devised to enclose and seal both the fusible member and the slit for housing the latter within a fuse box having a hermetic structure. This measure has sometimes put the interior of the fuse box under a high pressure which requires in may cases to construct the fuse box to withstand mechanically high pressures and particularly for the high current capacity type.
In addition, repeated flows of current through the conventional type of current limiting fuses have generally caused the fusible member to be subjected to repeated thermal stresses because it is repeatedly and alternately expanded and contracted due to the heat cycles thereof. This might result in the occurrence of an abnormal disconnection fault. In order to avoid such a fault, conventional current limiting fuses have been constructed so that the thermal stresses as above described are absorbed by forming the fusible member into a spiral or a zigzag. The excessive bending of the fusible member has resulted in the disadvantage that a decrease in reliability follows. This is not desirable.
Accordingly, it is an object of the present invention to provide a new and improved slit type current limiting fuse having the current limiting characteristic more excellent than that previously obtained by increasing sufficiently an arc voltage and an arc resistance of an electric arc struck upon the blowout of a fusible member involved.